DYNAMICAL GEOLOGY. 1JJ 



crumpled. The work effected by the compression and 

 movement of the rocks is transmuted into heat, and under 

 local conditions of concentration of the movements or sudden 

 cessation and relief of pressure, the temperature of the crushed 

 rocks may arrive at the point of actual fusion. If interstitial 

 or descending surface-water be absorbed by the glowing 

 rock-masses in sufficient quantity, its conversion into steam at 

 any moment of diminished pressure may give origin to 

 explosive volcanic phenomena at the surface. These are the 

 general arguments in Mallet's theory of volcanicity, which 

 was strengthened by the author's elaborate series of experi- 

 mental researches on the stresses required to crush different 

 varieties of rock, and the amount of heat that would be 

 produced in each case by this mechanical means. 



Mallet's theory has been contested by Justus Roth in 

 Germany and by Poulett-Scrope and Fisher in England. But 

 certain ideas in it, such as the steady contraction of the 

 earth's nucleus and its tendency to shrink away from an 

 unequally yielding crust, have proved distinctly valuable in the 

 consideration of the earth's physics, and have been variously 

 applied by later authors. 



Most geologists at present look sceptically upon any theory 

 which derives volcanic action from the conversion of 

 dynamical energy into heat during crust-movements. Present 

 opinion associates volcanic phenomena with the primitive 

 internal heat of the earth, and supposes rock-magma to be 

 embodied in a state of fusion within the earth's mass. This 

 was likewise the broad conception of volcanicity which was 

 held by the ancient philosophers, and by Athanasius Kircher, 

 Steno, Buffon, Dolomieu, Spallanzani, Faujas de Saint-Fond, 

 Von Humboldt, Von Buch, Poulett-Scrope, Daubeny, and 

 Lyell. 



The -actual protrusion ot subterrestrial magmas into the 

 earth's crust or at the surface was attributed by Cordier, 

 Constant Prevost, and Dana to the cooling of the earth's crust 

 and the pressure which it therefore exerts upon the nuclear 

 mass. Professor Suess has applied the distinctive term of 

 " batholite " to an older massive protrusion of magma 

 solidified as coarse crystalline rock in the deep horizons of the 

 crust. In 1888, the same geologist in his famous work, Der 

 Antlilz der Erde, discusses the conditions which determine 

 the particular form of igneous protrusion, whether as deep- 



